The present invention relates to investment casting of metallic materials and to fugitive patterns for use in the investment casting process, pattern assemblies and apparatus for assembling patterns.
In the well known xe2x80x9clost waxxe2x80x9d process of investment casting, a fugitive or disposable wax pattern is made by injection molding melted wax in a die corresponding to the configuration of the article to be cast. Typically, each wax pattern includes integral wax gating. A plurality of such molded wax patterns then are joined to a common wax runner bar by wax welding the gating to the runner bar. A frusto-conical or other wax pour cup typically is wax welded to the runner bar to complete the pattern assembly. The pattern assembly is invested in a ceramic shell mold by repeatedly dipping the pattern in a ceramic slurry, draining excess slurry, stuccoing with coarse ceramic particles or stucco, and air drying until a desired thickness of a ceramic shell mold is built-up on the pattern assembly. The pattern assembly then is removed from the green shell mold typically by heating the shell mold to melt out the pattern assembly, leaving a ceramic shell mold which then is fired at elevated temperature to develop appropriate mold strength for casting a molten metal or alloy.
In the past, the wax patterns have been wax welded manually to the wax runner bar. Such manual wax welding is disadvantageous in that it is time consuming and costly as a result and also produces pattern assemblies that exhibit high variability from one pattern assembly to the next with respect to dimensional locations of the patterns on the runner bar and the strength of the wax weld between the pattern gating and the runner bar from one pattern to the next on the runner bar. Improper pattern positioning on the runner bar and breaking off of some patterns at the wax weld can occur.
An object of the invention is to provide a fugitive pattern and method of making a fugitive pattern assembly for use in the lost wax precision investment casting process that overcome the above disadvantages.
Another object of the invention is to provide apparatus for manipulating a fugitive pattern to position it relative to another component of a pattern assembly.
Another object of the invention is to provide an investment casting having features adapted to be engaged by a manipulator.
The present invention provides in one embodiment a fugitive pattern of an article to be investment cast wherein the pattern includes a plurality of locators disposed in an array to provide a datum reference system by which the pattern can be held and positioned by a manipulator, such as for example a gripper device pursuant to another embodiment of the invention coupled to a computer controlled robotic motion device, for assembly with another component of a pattern assembly. Preferably, the datum locators are located on a portion of the pattern that will be removed from the final metallic casting made to replicate the pattern. For example, the datum locators preferably are located on a gating region of the fugitive pattern such that the metallic gating is cut-off from the final casting in a one step cut-off operation.
In a particular embodiment of the invention, a plurality of locator embossments on the fugitive (e.g. wax) pattern define a reference plane that is positioned parallel to a plane of orientation determined for the surface of a fugitive (e.g. wax) support member, such as a runner bar. Prior to placing the attaching surface of the fugitive pattern in proximity to and facing the surface of the fugitive support member, a sensor on the gripper device is moved over the surface of the support member by the robotic motion device to determine planar orientation of a particular area of the support member surface where each successive pattern is to be attached, which planar orientation is stored in robot control unit memory. When the pattern attaching surface is then placed proximate and facing the area of the surface of the support member, the gripper device is manipulated by the robotic arm to orient the pattern attaching surface so as to have substantially the same orientation as the sensed and stored planar orientation.
The present invention provides in another embodiment a method of making a fugitive pattern assembly by placing an attaching surface of a fugitive pattern in proximity to and facing a surface of a fugitive support member, such as for example, a wax runner bar. A heating device is placed between the attaching surface of the pattern and the surface of the support member to melt a puddle of the fugitive material on the surface of the support member and soften but not melt the pattern attaching surface. The heating device is removed. The pattern and support member then are relatively moved to contact the pattern attaching surface and the melted puddle, which is solidified during such contact to form a joint therebetween. Preferably, the pattern is manipulated by a robotic device in a manner that the attaching surface of the pattern is first moved a preselected distance below the surface of the melted puddle and then moved in an opposite direction a lesser preselected distance to form a smooth filleted corner at the joint between the pattern and the support member.
The invention provides in another embodiment a gripper device for gripping a fugitive pattern to accurately position it relative to another component of a pattern assembly. The gripper device preferably includes a position sensing device and heating device that is movable in a manner to melt a puddle of pattern material on the component to be assembled to the pattern.
The fugitive pattern having the above locators thereon is used in the lost wax investment casting process to cast an article that includes a plurality of integral locators disposed in an array to provide a datum reference system by which the cast article can be held and positioned by a manipulator for further processing.
Objects and advantages of the invention will become more readily apparent from the following detailed description.